Planar waveguide optical isolator in thin silicon-on-isolator (SOI) structure

ABSTRACT

A planar optical isolator is formed within the silicon surface layer of an SOI structure. A forward-directed signal is applied to an input waveguiding section of the isolator and thereafter propagates through a non-reciprocal waveguide coupling region into an output waveguide section. A rearward-directed signal enters via the output waveguide section and is thereafter coupled into the non-reciprocal waveguide structure, where the geometry of the structure functions to couple only a small amount of the reflected signal into the input waveguide section. In one embodiment, the non-reciprocal structure comprises an N-way directional coupler (with one output waveguide, one input waveguide and N−1 isolating waveguides). In another embodiment, the non-reciprocal structure comprises a waveguide expansion region including a tapered, mode-matching portion coupled to the output waveguide and an enlarged, non-mode matching portion coupled to the input waveguide such that a majority of a reflected signal will be mismatched with respect to the input waveguide section. By cascading a number of such planar SOI-based structures, increased isolation can be achieved—advantageously within a monolithic arrangement.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Application No.60/526,801, filed Dec. 4, 2003.

TECHNICAL FIELD

The present invention relates to the field of silicon-on-insulator (SOI)structures and, more particularly, to the formation of a planar opticalisolator component within an SOI structure.

BACKGROUND OF THE INVENTION

Optical isolators are well-understood in the art and find uses inoptical communication systems, sensors and the like. The purpose of anoptical isolator is to eliminate unwanted or reflected optical signalsfrom interfering with a desired optical function. For example, anisolator may be inserted in an optical signal path between a distributedfeedback (DFB) laser and an optical fiber. Without the isolator,unwanted optical signals (i.e., reflections) from the optical fiberwould couple back into the DFB laser and adversely affect itstransmitted optical spectrum. By including an isolator in this design,unwanted reflected signals are absorbed by the isolator and do not reachthe laser. Non-planar optical isolators typically employ birefringentcrystal plates (e.g., rutiles), half-wave plates and latching garnets ornon-latching garnets with external magnets (hereinafter referred to ingeneral as “Faraday isolators”).

A planar, waveguide-based isolator has been developed in the opticaldomain for integration with a semiconductor light emitting diode, suchas Fabry-Perot or ring laser diodes, as disclosed in U.S. Pat. No.5,463,705, issued to R. Clauberg et al. on Oct. 31, 1995. In theClauberg et al. arrangement, a waveguide directional coupler is formedwithin one layer of a III–V material system (e.g., GaAs, InP, etc.) soas to be integrated with a III–V based light emitting diode. Thedirectional coupler is formed as a rib waveguide directional couplingstructure, where at least one branch of the coupler includes an“absorber means” to collect a reflected, unwanted optical signal andreduce its further propagation through the coupler (i.e., “isolates” thereflected signal). While the Clauberg et al. waveguide coupler includingat least one isolating branch may be integrated with a III–V lightemitted device to increase optical efficiency, the emitter/isolatorremains as a discrete component that must ultimately be combined withvarious other optical and electronic elements that are not well-suitedto formation in III–V materials.

Indeed, as the complexity of optical system design increases, the needto monolithically integrate multiple optical and electrical functionsonto a single material substrate is becoming a necessity, in order toreduce the size and cost of the optical system. It has recently beenrecognized that the materials, processes and fabrication techniques usedin the production of silicon-based electronic devices can be adapted forthe processing of optical elements. Advantageously, it has now beenshown that various optical elements and their associated electronicactivation devices can be integrated within the same substrate. The basematerials system of choice for this simultaneous integration ofelectronics and optics is silicon-on-insulator (SOI), where theelectronics have been well-characterized for many decades using CMOSprocessing technology, and the optics can be reduced to extremely smallsizes as a result of the inherently high index optical guides that canbe fabricated in SOI.

The monolithic integration of a conventional Faraday isolator into aplanar silicon integrated circuit fabrication process is not consideredto be practical. Indeed, the conventional Faraday isolator device relieson the use of the magneto-optic effect to rotate the light and provideisolation. Silicon does not exhibit any such magneto-optic effect.Various materials that do exhibit a magneto-optic effect are notconsidered to be compatible with conventional silicon semiconductorprocessing technology.

Thus, a need remains in the art for an optical isolator structure thatis compatible with the silicon processing techniques used in theformation of SOI-based optoelectronic devices.

SUMMARY OF THE INVENTION

The need remaining in the prior art is addressed by the presentinvention, which relates to the field of silicon-on-insulator (SOI)structures and, more particularly, to the formation of a monolithicarrangement of a planar optical isolator and associated optic/electroniccomponents within an SOI structure.

In accordance with the present invention, an optical waveguidingstructure is formed within a silicon surface layer (strained or notstrained) of an SOI structure (hereinafter referred to as the “SOIlayer”) to include a non-reciprocal, highly directional couplingarrangement. In the forward direction of propagation, an optical signalintroduced into an input section of the optical waveguiding structurewill thereafter propagate through the non-reciprocal, highly directionalcoupler essentially unaffected and thereafter propagate along an outputsection of the optical waveguiding structure. In the reverse direction,a reflected optical signal will be affected by the non-reciprocal,highly directional coupler such that most of the unwanted, reflectedsignal is intercepted and dissipated, while only a relatively smallportion of the reflected signal will be coupled back into the inputsection of the waveguide.

In one embodiment of the present invention, the non-reciprocal highlydirectional coupler comprises a planar waveguide N-waysplitter/combiner. The forward-directed signal is coupled into one ofN−1 “input” waveguides, propagates through a coupling region and is thencoupled into the “output” waveguide. In the reverse direction, areflected signal coupled into the output waveguide will propagatethrough the coupling region and be “split” among each of the N−1 inputwaveguides. In accordance with this embodiment of the present invention,each of these N−1 input waveguides (except the input waveguide) isterminated by an isolating element to prevent further unwantedpropagation of this reflected signal. In one exemplary configuration, asimple Y-combiner/splitter waveguide may be used as the non-reciprocal,highly directional coupler. In this case, the angle between thewaveguides of the input port and the reflecting port should berelatively small (typically less than 50°) in order to obtain therequired directionality. The Y-combiner may be formed as a symmetriccombiner/splitter or an asymmetric combiner/splitter.

The isolating element may comprise a light absorbing element, such as ametal or silicide strip disposed over the waveguiding layer. In thiscase, the light absorbing element may also function as a photodetector,measuring the amount of reflected signal. Alternatively, a lightdiffusing element may be introduced into the isolating waveguide, suchas a diffraction grating or tapered waveguide. Indeed, an isolatingelement of the present invention may be formed as a combination of alight absorbing element and a light radiating element. Any of theseisolating arrangements are advantageously compatible with conventionalplanar CMOS processing techniques, allowing for various other desiredoptical and electrical components to be integrated with the opticalisolator and form a monolithic arrangement.

In an alternative embodiment of the present invention, thenon-reciprocal, highly-directional optical isolator comprises an opticalwaveguide signal expansion region disposed between an optical inputwaveguide region and an optical output waveguide region. Particularly,the signal expansion region is formed, using conventional, well-knownplanar CMOS processing techniques to exhibit a geometry such that thelarger end of the expansion region is adjacent to the input waveguideregion, and then tapers (symmetrically) in the direction of the outputwaveguide section. With this structure, a transmitted signal propagatingalong the input section of the waveguide will first encounter the largerend of the expansion section and begin to “fill” this section. Theexpansion section is configured to taper towards the output couplingregion such that the transmitted signal will maintain its mode andultimately be coupled into the output waveguide region. In the reversedirection, a “reflected” signal will propagate through the outputwaveguide region and enter the signal expansion region at the taperedend. The reflected signal will continue to expand and fill the fullextent of the expansion region. Inasmuch as the input waveguide regionis in contact with this larger end of the expansion region, only aminimal amount of the reflected signal will enter the input couplingregion. Indeed, most of the (unwanted) reflected signal will reflectaround in the expansion region and be ultimately lost throughdissipation.

An advantage of the SOI-based, planar optical isolator of the presentinvention is that an additional, patterned polysilicon layer may beformed over certain portions of the waveguide structure and improve theamount of isolation that may be achieved. By virtue of using well-knownplanar CMOS processing technology, the location, size and shape of thedesired polysilicon layer can be well-controlled.

Indeed, by virtue of utilizing the SOI structure, a plurality ofSOI-based planar isolators (and associated components) of the presentinvention may be disposed in a cascaded, monolithic arrangement within asingle SOI substrate such that the isolation is improved with each stageadded to the structure. Thus, if a single stage isolator is capable ofproviding 3 dB isolation (presuming a 50:50 split between the input portand a single reflecting port), a cascaded arrangement of ten suchisolators will provide approximately 30 dB of isolation.

Other and further advantages and arrangements of the present inventionwill become apparent during the course of the following discussion andby reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings,

FIG. 1 is an isometric view of an exemplary single stage, SOI-basedplanar isolator formed in accordance with one embodiment of the presentinvention;

FIG. 2 is a top view of the arrangement of FIG. 1, illustrating itsperformance in the forward (transmitting) direction;

FIG. 3 is a top view of the arrangement of FIG. 1, illustrating itsperformance in the rearward (isolating) direction;

FIG. 4 is a top view of another arrangement of the first embodiment of asingle stage, SOI-based planar isolator of the present invention,utilizing a multiple arm splitter/combiner configuration;

FIG. 5 is a top view of an alternative realization of the firstembodiment of the present invention, utilizing an optical scatteringelement to provide the desired optical isolation;

FIG. 6 contains an isometric view of an embodiment of the presentinvention, utilizing a polysilicon structure disposed over the SOIlayer;

FIG. 7 contains a top view of an exemplary multiple armsplitter/combiner configuration of the inventive isolator that includesthe use of a patterned polysilicon layer;

FIG. 8 is a top view of an exemplary three-stage SOI-based planarisolator of the first embodiment of the present invention;

FIG. 9 contains graphical representations of the forward lightpropagation (FIG. 9A) and the attenuation of the back-scattered light(FIG. 9B);

FIG. 10 contains an isometric view of a second embodiment of the presentinvention, utilizing an optical waveguiding expansion region to providefor isolation of an input SOI-based waveguide from reflected signals;

FIG. 11 is a top view of the embodiment of FIG. 10 in the forward,transmitting mode;

FIG. 12 is a top view of the embodiment of FIG. 10 in the rearward,reflecting mode;

FIG. 13 is an isometric illustration of an alternative configuration ofthe second embodiment, including a patterned polysilicon layer over theSOI-based waveguide expansion region;

FIG. 14 is a top view of the arrangement of FIG. 13; and

FIG. 15 is a simplified diagram of a three-stage planar, SOI-basedoptical isolator formed in accordance with the second embodiment of thepresent invention.

DETAILED DESCRIPTION

FIG. 1 contains an isometric view of an exemplary SOI-based planaroptical isolator 10 formed in accordance with the present invention.Optical isolator 10 is formed within a silicon-on-insulator (SOI)structure 12 comprising a silicon substrate 14, an insulating dielectriclayer 16 (usually comprising SiO₂) and a surface single crystal siliconlayer 18 (typically referred to in the art as the “SOI layer”). In mostcases, SOI layer 18 will comprise a thickness of less than one micron.In some cases, the surface silicon layer may comprise a “strained”silicon layer, where the lattice structure of the silicon has beenmodified. Although not particularly illustrated in FIG. 1, or any of thefollowing drawings, it is to be understood that in actual use, theassociated opto-electronic components are formed within the same SOIstructure 12, thus forming an efficient, monolithic optical systemarrangement.

In accordance with the present invention, optical isolator 10 includes anon-reciprocal, highly directional coupler 11 formed, at least in part,within SOI layer 18 using well-known planar CMOS processing technology.For the particular embodiment of FIG. 1, highly directional coupler 11comprises an N-way optical waveguide splitter/combiner (where in thearrangement of FIG. 1, N=2 and the coupler comprises a Y-couplergeometry). It is to be understood that this geometry is exemplary only,and various other types of highly directional coupling arrangements(i.e., an “input waveguide section”, “output waveguide”, at least one“isolating waveguide section”) may be formed that exhibit the isolatingability of this particular embodiment of the present invention. As shownin FIG. 1, optical isolator 10 comprises an input port 20 for receivingan input optical signal. In most cases (particularly where a sub-micronSOI layer is used), the propagating signal will comprise a single modesignal. The optical signal thereafter propagates along an inputwaveguide section 22 and enters a coupling region 24. An outputwaveguide 26 is coupled to coupling region 24 and thus supports thetransmission of the forward-propagating optical signal toward an outputport 28.

Isolation in this arrangement is provided, in accordance with thepresent invention, by the utilization of an isolating element 30disposed over an isolating waveguide 32, where isolating waveguide 32 isalso coupled to coupling region 24, as shown in FIG. 1. In theembodiment of FIG. 1, isolating element 30 comprises an opticallyabsorbing material 34 (such as a metal, germanium, SiGe, highly-dopedsilicon or silicide) that functions to collect the propagating opticalsignal. As will be discussed below in association with FIG. 5, anoptical radiating arrangement (such as a diffraction grating or taper)may be formed within isolating waveguide 32 and used to disperse thereflected optical signal propagating along isolating waveguide 32.

Advantageously, well-known planar CMOS processing technologies may beused to form the requisite isolating element of the present invention.In particular, the deposition of metals and/or the formation ofsilicides through the reaction of the SOI layer with a deposited metalare well understood. Indeed, as discussed above, the ability to useconventional planar CMOS processing technology allows for the planaroptical isolator of the present invention to be incorporated withvarious other opto-electronic elements into a monolithic, SOI-basedarrangement.

As mentioned above, silicon does not have a magneto-optic effect.Therefore, the optical isolator of the present invention is based uponthe non-reciprocal geometry of the highly direction coupler (in contrastto the non-reciprocal optical properties of a magneto-optic device). Inparticular, it is preferred that the angle between input waveguide 22and isolating waveguide 32 be no more than 50°. Advantageously, thepolarization orientation of light exiting isolator 10 will be the sameas the polarization orientation of the light entering isolator 10. Thiscontinuity in polarization is desirable for silicon waveguides, wherecertain optical modes (TE or TM) propagate with less optical loss thanother modes. The dependencies on temperature and wavelength associatedwith magneto-optic materials and Faraday rotation, as used in prior artisolators, are not present in the inventive SOI-based planar opticalisolator structure. Moreover, as mentioned above, the use of anSOI-based optical isolator structure allows for integration with variousother SOI-based components (optical and/or electrical), where the planargeometry of the inventive isolator is also considered advantageous froma performance point of view.

When reflected light enters the boundary of isolating element 30,reflections will again occur (due to the mismatch between theabsorbing/scattering region and the waveguide region). These reflectionscan be minimized by forming a transition region that provides a moregraceful transition in optical mode coupling between isolating waveguide32 and isolating element 34. FIG. 3 (which will be described in moredetail hereinbelow) illustrates an exemplary arrangement of the presentinvention that includes an input tapered region 36 with isolatingelement 34, where the taper will provide for the desired transition ineffective index.

FIG. 2 is a top view of SOI-based planar isolator 10 of FIG. 1,illustrating in particular the propagation of an optical signal in theforward (transmitted) direction. As shown, an input optical signal iscoupled via input port 20 into input waveguide 22. The signal passesthrough coupling region 24 and thereafter into output waveguide 26,exiting isolator 10 at output port 28. The isolation path for isolator10 is illustrated in FIG. 3. As shown, a “reflected” optical signalenters isolator 10 at output port 28 and thereafter propagates alongoutput waveguide 26 and enters coupling region 24. In this case,coupling region 24 will function as a Y-splitter, coupling the reflectedsignal into both input waveguide 22 and isolating waveguide 32. Asdiscussed above, the angle Φ between input waveguide 22 and isolatingwaveguide 32 needs to be relatively small, in order to provide thedesired high directional coupling effect of combining region 24. Inparticular, maintaining Φ to be less than 50° is considered to bepreferable. In accordance with the present invention, the portion of thereflected optical signal propagating along isolating waveguide 32 willbe captured by isolating element 30, effectively preventing furtherpropagation of that portion of the reflected signal.

In general, and as mentioned above, isolating element 30 may comprise anoptical absorbing element, an optical radiating element, or anycombination thereof. In the particular embodiment illustrated in FIG. 3,isolating element 30 comprises an absorbing material 34 that is disposedover (and/or within) a portion of isolating waveguide 32, whereabsorbing material 34 functions to capture the optical signal andprevent further propagation. Absorbing material 34 may comprise a metal(such as, for example, titanium, tantalum, aluminum, copper, etc.), asilicide (such as, for example, cobalt silicide, titanium silicide, etc), or any appropriate silicon processing-compatible material (such as,for example, germanium, highly-doped silicon, SiGe, etc.). Indeed, anysuitable material that functions to absorb a propagating lightwave maybe used. As further shown in FIG. 3, absorbing material 34 (in thisparticular arrangement) is formed to include an input taper 36, wheretaper 36 reduces secondary reflections back along reflecting waveguide32 by introducing a transitional change in effective index alongwaveguide 32 (instead of an “abrupt” change that would be associatedwith the disposition of a conventional layer ofmetal/silicide/conducting material over isolating waveguide 32).Advantageously, absorbing material that produces an electric currentproportional to the amount of absorbed light can be configured as anoptical photodetector and used to measure the amount of absorbed light.Indeed, by virtue of the ability to include electronics with the planar,SOI-based isolator of the prior art, additional electronic circuitry(for example, a feedback circuit) may be integrated with the detectorand used to monitor the amount of reflected signal to control/adjustvarious other optical system parameters.

In the embodiment as illustrated in FIGS. 1–3, highly directionalcoupler 11 comprises the form of a Y-combiner, and is presumed to affecta 50:50 split of an optical signal in the reverse, reflected direction.Presuming also that a single mode optical signal is propagating throughthe coupler, a 3 dB amount of isolation will be introduced into thereflected signal. Theoretically, any other desired split ratio may beused. For example, a 10:90 split of an optical signal in the reverse,reflected direction would produce 10 dB of isolation into the reflectedsignal. Alternatively, an improvement in the amount of isolationprovided by a single stage, SOI-based planar isolator of this particularN-way optical waveguide splitter/combiner of the present invention canbe accomplished by using a multiple arm planar highly directionalcoupler.

FIG. 4 contains a top view of one such multiple-arm planar isolator 40of the present invention, including an input waveguide 42, an outputwaveguide 44, a coupling region 46, and a pair of isolating waveguides48, 50. Referring to FIG. 4, isolator 40 is shown as having a separateone of isolating waveguides 48, 50 disposed on either side of inputwaveguide 42, with a predetermined angular displacement θ between eachreflecting waveguide 48, 50 and input waveguide 42. First isolatingwaveguide 48 is illustrated as including a first isolating element 52and second isolating waveguide 50 is illustrated as including a secondisolating element 54. Isolation elements 52, 54 may comprise anoptically radiating structure, an optically absorbing structure, or acombination of both. Moreover, as discussed above, each isolatingelement may further comprise an input tapered section to preventsecondary reflections, these input tapered sections illustrated ascomponents 56, 58 in FIG. 4.

In this particular embodiment of the present invention, an unwanted,reflected signal propagating along output waveguide 44 will pass throughadiabatic coupling region 46 and be coupled into both isolatingwaveguides 48, 50, as well as input waveguide 42. By controlling thegeometry of isolating waveguides 48, 50, as well as the angulardisplacement θ between the isolating waveguides and the input waveguide,a significant portion of the unwanted reflected signal can be directedinto isolating waveguides 48, 50 and isolated from further backwardpropagation along input waveguide 42 by isolating elements 52, 54.Importantly, coupling region 46 is configured to adiabatically taperbetween the input/ reflecting waveguides and the output waveguide so asto maintain mode matching between the forward-directed transmittingsignal propagating along input waveguide 42 and output waveguide 44.

As mentioned above in association with FIGS. 1–3, isolating element 30may take the form of a grating structure, tapered waveguide, or anyother dispersive element that would function to radiate the light signalentering isolating element 30. FIG. 5 is a top view of an alternativearrangement of this N-way optical waveguide splitter/combiner embodimentof an SOI-based planar optical isolator 51 of the present invention,where like elements from the configuration of FIGS. 1–3 contain likereference numerals in FIG. 5. In this case, isolating element 30 takesthe form of a diffraction grating 53 formed within isolating waveguide32. Since SOI layer 18 comprises silicon, any suitable technique foretching or otherwise patterning SOI layer 18, as well known from theCMOS processing technology, may be used to form the desiredconfiguration of diffraction grating 53 directly within reflectingwaveguide 32. Indeed, the pitch, period and/or other gratingcharacteristics of grating 53 may be controlled by CMOS processingtechniques (etching, for example) so as to introduce a gradual change inthe effective index between isolating waveguide 32 and isolating element30.

In some SOI-based structures, a polysilicon layer is utilized, inassociation with the SOI layer (perhaps a strained silicon SOI layer),to create the waveguiding structure. Additional isolation may beachieved, in accordance with the present invention, by using a patternedpolysilicon layer disposed on selected portions of the variousabove-described isolator arrangements. FIG. 6 illustrates an exemplaryoptical isolator 60 of the present invention that includes a polysiliconstructure. As shown, optical isolator 60 is formed using an SOIstructure 62 including a silicon substrate 64, an isolating layer 66 andSOI layer 68 (where SOI layer 68 is generally less than one micron inthickness). A highly directional Y-coupler 70, used to provide isolationin accordance with the present invention, is formed to include an inputwaveguiding structure 72, an output waveguiding structure 74, an opticalcombining region 76 and an isolating waveguiding structure 78. In thisparticular embodiment of the present invention, highly directionalcoupler 70 comprises a patterned polysilicon layer 80 disposed over aportion of isolating waveguiding structure 78, as shown. The presence ofpolysilicon layer 80 will modify the amount of reflected optical signaldirected into isolating waveguiding structure 78 and thus improve theisolation properties of the arrangement. Moreover, the particularconfiguration of polysilicon layer 80 will have a minimal effect on thedesired, forward-directed propagating optical signal.

As with the embodiments discussed above, isolation of an unwanted,reflected signal is provided by including an isolating element 82 over(or within) isolating waveguiding structure 78. Indeed, similar to theabove arrangements, isolating element may comprise an optical absorbingcomponent, an optical radiating component, or any combination thereof.The use of well-known CMOS processing technology allows for patternedpolysilicon layer 80 to easily be formed, and proportioned to providethe desired improvement in isolation.

An exemplary 3-way optical waveguide splitter/combiner isolator 90 ofthe present invention, including a patterned polysilicon structure, isshown in a top view in FIG. 7. In this case, isolator 90 comprises aninput waveguiding section 92, output waveguiding section 94, and a pairof isolating waveguide sections 96 and 98, with isolating waveguidesection 96 including an isolating element 93 and isolating waveguidesection 98 including an isolating element 95. A patterned polysiliconlayer in the form of symmetric regions 97 and 99 is shown as disposedover isolating waveguide sections 96 and 98, respectively. As with thearrangement discussed above in association with FIG. 6, the presence ofpolysilicon regions 97, 99 will cause a larger amount of reflected(unwanted) optical signal energy to be guided into isolating waveguidingsections 96, 98, further increasing the amount of isolation achievablewith this structure. It is to be understood that the particular geometryof regions 97, 99 is exemplary only, and various other polysiliconregions (or a single region) may be used to improve the isolationproperties of the inventive structure.

FIG. 8 illustrates an alternative, multi-stage arrangement of the N-wayoptical waveguide splitter/combiner embodiment of the present invention.Advantageously, the utilization of an SOI-based isolator employingwell-known planar CMOS fabrication technology allows for multiple,essentially identical isolator stages to be formed and coupled togetheras a monolithic structure. As shown in FIG. 8, multi-stage SOI-basedplanar optical isolator 100 comprises three separate isolation stages,denoted as 110-1, 110-2 and 110-3 in the drawing. Each stage is formedin an exemplary Y-combiner configuration as discussed above (any otherappropriate N-way splitter/combiner geometry may be used), where in the“forward” direction, the optical signal exiting output port 112-1 fromstage 110-1 is applied as the input signal (at input port 118-2) ofisolator stage 110-2, and so on. In the reverse (isolating) direction,each highly directional coupler 116-1, 116-2 and 116-3 will function toremove a predetermined portion (for example, half) of any reflectedsignal, where the removed portion is thereafter intercepted by isolatingelement 114-1, 114-2 and 114-3. Since each isolator in this particularembodiment will add (for example) a 3 dB level of isolation, thecascaded arrangement as illustrated in FIG. 7 will provide a 3 dB dropin reflected/isolated signal at each stage. Thus, if a 10-stage isolatorwere to be utilized, a 30 dB drop in reflected signal could be expected.An even higher degree of isolation may be accomplished, for example, byutilizing a multi-stage isolator utilizing the multiple armconfiguration discussed above in association with FIG. 4 or,alternatively, a “poly-loaded” arrangement as shown in FIGS. 6 and 7.

FIG. 9 is a graphical representation of a simulation of a light signalpropagating along the multi-stage isolator 100 of FIG. 8, where FIG. 9Aillustrates the propagation of light in the forward direction. FIG. 9Billustrates the propagation of reflected light in the reverse direction.As shown, the presence of a highly directional coupler, in combinationwith an isolating element, serves to significantly reduce the presenceof a reflected signal at input port 20.

As mentioned above, the present invention in its most general form isbased on the realization that a planar waveguide structure may be formedin an SOI system to as to exhibit a non-reciprocal, highly directionalgeometry and, as a result, provide a passive optical isolation function.The embodiment as described in associated with above FIGS. 1–9 areassociated with using an N-way optical waveguide splitter/combiner asthe non-reciprocal, highly directional coupler. Various other geometrieshave been contemplated and are capable of providing the desirednon-reciprocity and optical signal isolation. FIGS. 10–12 illustrate onesuch other embodiment of a non-reciprocal, highly directional, planarSOI-based optical isolator formed in accordance with the presentinvention.

FIG. 10 contains an isometric view of an exemplary SOI-based planaroptical isolator 200 formed in accordance with the present invention.Optical isolator 200 is formed within a silicon-on-insulator (SOI)structure 210 comprising a silicon substrate 212, an insulatingdielectric layer 214 (usually comprising SiO₂) and a surface singlecrystal silicon layer 216 (typically referred to in the art as the “SOIlayer”). In most cases, SOI layer 216 will comprise a thickness of lessthan one micron, and may be formed as a strained silicon layer. Asshown, SOI layer 216 has been patterned, using conventional CMOSprocessing technology, to form a non-reciprocal, highly directionalwaveguide expansion section 218, input waveguiding section 220 andoutput waveguiding section 222. FIGS. 11 and 12 contain top views ofoptical isolator 200 in the “transmitting” and “isolating” modes,respectively.

In accordance with the present invention, expansion section 218 isformed between input waveguiding section 220 and output waveguidingsection 222 so as to function as a signal expander/dissipater forreflected (unwanted) signals propagating from output waveguide section222 towards input waveguiding section 220. Expansion section 218 isformed to include an enlarged region 224 adjacent to input waveguidingsection 220 and a tapered transition region 226 adjacent to outputwaveguiding section 222. Referring to FIGS. 10 and 11, when atransmitted optical signal is propagating along input waveguidingsection 220 (in the direction as indicated in FIG. 11), the opticalsignal will encounter enlarged region 224 of expansion section 218, andbegin to expand in mode to fill the confines of enlarged region 224.Thereafter, however, the propagating signal will enter transitionsection 226 and gradually diminish in overall size so as to provideessentially complete coupling of the propagating signal into outputwaveguiding section 222. Thus, in the forward direction, the completeoptical signal passes from the input to the output.

The isolating function of this embodiment of the present invention maybe understood with reference to FIGS. 10 and 12. In this case, areflected (unwanted) signal is propagating in the backward directionalong output waveguiding section 222 (as shown by the arrow in FIG. 12).The reflected signal then enters transition region 226, which functionsto slowly increase the mode size of the reflected signal, continuing toexpand the mode diameter as the signal enters enlarged region 224 ofexpansion section 218. Therefore, as shown in particular in FIG. 12,when the reflected optical signal encounters the interface between inputwaveguiding section 220 and enlarged region 224, the majority of thesignal will be “lost” to improper matching, permitting only a smallfraction of the optical signal, as shown, to be coupled into inputwaveguiding section 220. The remainder of the signal will be reflectedat the mismatched index barrier between the silicon expansion section218 and the material used to planarize the SOI structure. In most cases,a blanket deposited layer of silicon dioxide is used for this planarize(as is common in conventional CMOS electronic circuits). The reflectedsignal continues to reflect off surfaces within enlarged region 224 andtransition region 226 until it is sufficiently dissipated. Preferably,tapered transition region 226 is formed to exhibit an adiabatic taper,so as to avoid any resonant effects to be generated over the wavelengthrange of interest.

This particular embodiment of the present invention, utilizing anin-line expansion section, thus provides a relatively large degree ofisolation without the need to include separate isolating elements (suchas absorbing or radiating elements) within the waveguiding structure.However, an even greater degree of isolation (with less worry aboutreflected signals within expansion section) can be had by including anisolating element (either an absorbing element or radiating element, asdiscussed above) with selection portions of expansions section 218.Other arrangements for dissipating the reflected signal at thetermination of enlarged region 224 may be used, such as forming a trenchregion along the perimeter of enlarged region 224.

As with the N-way optical waveguide splitter/combiner embodimentdiscussed above, a patterned polysilicon layer may be added to thestructure of FIG. 10 to provide for an improved degree of isolation.FIG. 13 is an isometric view of an exemplary arrangement of theexpansion waveguide section embodiment of the present invention(utilizing the same reference numerals as FIG. 10), with the inclusionof polysilicon regions 228, 230 over selected portions of expansionsection 218. Polysilicon regions 228 and 230 are patterned so as to besymmetric with respect to the optical axis of a forward-propagatingsignal so as to not interact with this signal or degrade thetransmissive properties of the invention optical isolator arrangement.FIG. 14 is a top view of the same arrangement. It is to be understoodthat the particular polysilicon pattern shown in FIGS. 13 and 14 isconsidered to be exemplary only. Various other polysilicon patterns maybe used to improve the isolator performance of the inventive structure.

As mentioned above, a considerable advantage of the SOI-based structureof the present invention is the ability to integrate the inventiveisolator with various other optical and electrical components in amonolithic arrangement. Further, the use of conventional CMOS processtechnology allows for an N-stage planar SOI-based optical isolator to beformed, where each stage is essentially the expanded-waveguide isolatordesign of FIG. 10. FIG. 15 illustrates a 3-stage isolator formed inaccordance with this aspect of the present invention. As shown, a set ofthree expansion sections 218-1, 218-2 and 218-3 are disposed in adistributed fashion along the length of the optical waveguide, where aninitial input waveguide 220 is illustrated as coupled to first expansionsection 218-1, and a final output waveguide 222 is illustrated ascoupled to the last expansion section 218-3.

It will become apparent to those skilled in the art that variousmodifications to the preferred embodiments of the present invention asdescribed herein can be made without departing from the spirit or scopethereof. Indeed, the subject matter of the present invention is intendedto be limited only by the scope of the claims, as appended hereto:

1. A planar optical isolator formed within a sub-micron thick surfacesilicon layer of a silicon-on-insulator (SOI) structure, the planaroptical isolator comprising: an input waveguiding region formed, atleast in part, within the sub-micron thick surface silicon layer anddefined as including an input port for coupling an input single modeoptical signal into the input waveguiding region; an output waveguidingregion formed, at least in part, within the sub-micron thick surfacesilicon layer and defined as including an output port for permitting apropagating single mode optical signal to exit the planar opticalisolator; and a non-reciprocal, highly directional optical couplingregions disposed between the input and output waveguiding regions fromtransmitting a forward-directed, transmitted single mode optical signalfrom the input waveguiding region to the output waveguiding region anddissipating a rearward-directed, reflected optical signal such that onlya relatively small portion of the reflected signal is coupled into theinput waveguiding regions, thus providing a substantial degree ofoptical isolation for signals propagating in the reverse direction,wherein the non-reciprocal, highly directional optical coupling regioncomprises an N-way optical waveguide splitter/combiner including: aplurality of N input waveguiding sections and a single outputwaveguiding section, with a single one of the N input waveguidingsections coupled to the isolator input waveguiding region, the remainingN−1 input waveguiding sections defined as isolating waveguidingsections, and the output waveguiding section coupled to the isolatoroutput waveguiding region; and a plurality of N−1 optical isolatingelements, each optical isolating element disposed along a portion of anassociated one of the N−1 isolating waveguiding sections such that arearward-directed, reflected optical signal is coupled into each one ofthe plurality of N input waveguiding sections and is dissipated alongthe plurality of N−1 isolating waveguiding sections by its associatedoptical isolating element such that only a relatively small portionpropagating along the input optical waveguiding region remains, whereineach isolating element comprises an optically radiative element formedwithin its associated isolating waveguiding region, the opticallyradiative element comprising a waveguide-based diffraction gratingstructure wherein the properties of the diffraction grating structureare configured to introduce an adiabatic change in effective indexbetween the isolating waveguiding section and its associated isolatingelement.
 2. The planar optical isolator as defined in claim 1 whereinthe optically radiative element comprises an outwardly tapered waveguidestructure.
 3. The planar optical isolator as defined in claim 1 whereinthe optically radiative element comprises an inwardly tapered waveguidestructure.
 4. The planar optical isolator as defined in claim 1 whereineach isolating element comprises an optically absorbing element formedwithin its associated isolating waveguiding region.
 5. The planaroptical isolator as defined in claim 4 wherein the optically absorbingelement includes a tapered input region configured to introduce anadiabatic change in effective index between the isolating waveguidingregion and the associated isolating element.
 6. The planar opticalisolator as defined in claim 4 wherein the optically absorbing elementis selected from the group consisting of: a metal, a silicide, germaniumand SiGe.
 7. The planar optical isolator as defined in claim 4 whereinthe optically absorbing element generates an electric current andfunctions as a photodetector to measure the absorbed optical signal. 8.The planar optical isolator as defined in claim 1 wherein each isolatingelement comprises a combination of an optically radiative element and anoptically absorbing element.
 9. The planar optical isolator as definedin claim 1 wherein the N-way optical waveguide splitter/combinercomprises a Y-geometry splitter/combiner having a pair of waveguidingregions, defined as the input waveguiding region and the isolatingwaveguiding section, the arms of the Y-geometry splitter/combiner havinga predetermined angular displacement between the input waveguidingregion and the isolating waveguiding section.
 10. The planar opticalisolator as defined in claim 9 wherein the predetermined angulardisplacement between the input waveguiding region and the isolatingwaveguiding section is no greater than 50°.
 11. The planar opticalisolator as defined in claim 9 wherein the optical waveguide couplingregion provides an essentially symmetric 50:50 split in reflectedsignals between the input waveguiding region and the isolatingwaveguiding section.
 12. The planar optical isolator as defined in claim9 wherein the optical waveguide coupling region provides an asymmetricsplit in reflected signals between the input waveguiding region and theisolating waveguiding section.
 13. The planar optical isolator asdefined in claim 12 wherein the optical waveguide coupling regionprovides an essentially 10:90 split in reflected signals between theinput waveguiding region and the isolating waveguiding section.
 14. Theplanar optical isolator as defined in claim 12 wherein the opticalwaveguide coupling region provides an essentially 20:80 split inreflected signals between the input waveguiding region and the isolatingwaveguiding section.
 15. The planar optical isolator as defined in claim1 wherein the N-way optical waveguide splitter/combiner comprises a3-way splitter/combiner having a single input optical waveguiding regionand a pair of isolating waveguiding sections, with one isolatingwaveguiding section disposed on either side of the input waveguidingregion.
 16. The planar optical isolator as defined in claim 1 whereinthe sub-micron thick surface silicon layer of the SOI structurecomprises a strained silicon layer.
 17. The planar optical isolator asdefined in claim 1 wherein the optical isolator further comprises apatterned polysilicon layer disposed over predetermined portions of thenon-reciprocal, highly directional optical coupling region so as tomodify the mode of the propagating reflected optical signal such that alarger portion of the reflected optical signal is directed away from theinput waveguiding region.
 18. The planar optical isolator as defined inclaim 1 wherein the N-way optical waveguide splitter/combiner comprisesan optical waveguiding expansion region disposed between the inputwaveguiding region and the output waveguiding region, the opticalwaveguiding expansion region including a tapered transition sectioncoupled to the output waveguiding region and an enlarged section coupledto the input waveguiding region, the combination of the taperedtransition section and enlarged section such that a forward-directedtransmitted signal is coupled from the input waveguiding region into theenlarged section and thereafter propagates through the taperedtransition region, in an inwardly tapered direction, so as to be coupledinto the output waveguiding region, and a rearward-directed, reflectedsignal is coupled from the output waveguiding region into the taperedtransition section, in an outwardly tapered direction and thereafterpropagates through the enlarged section so as to further increase inmode size such that only a relatively small portion of the reflectedsignal is coupled into the input waveguiding region.
 19. The planaroptical isolator as defined in claim 18 wherein the optical waveguidingexpansion region is formed so as to be substantially symmetric about thepropagation direction of an optical signal.
 20. The planar opticalisolator as defined in claim 18 wherein the isolator further comprises apatterned polysilicon layer disposed over the optical waveguidingexpansion region so as to increase the amount of reflected opticalsignal directed out of the signal path associated with the inputwaveguiding region.
 21. The planar optical isolator as defined in claim18 wherein the isolator further comprises an isolating element disposedover a portion of the optical waveguiding expansion region so as todissipate the reflected optical signal.
 22. The planar optical isolatoras defined in claim 21 wherein the isolating element comprises anoptically absorbing element.
 23. The planar optical isolator as definedin claim 21 wherein the isolating element comprises an opticallyradiative element.